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02-20-2015, 09:33 PM   #1
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Tokina 90mm 2.5 problems

I bought a Bokina the other day with some issues. The seller was not aware of what they were, suffice to say it was obvious that the aperture blades were a little sticky and there was some 'haze' in the rear element immediately adjacent to the blades.

I took a punt knowing that the purchase price was exceptionally good for this item and my ability to open up and clean was equal to the task. Sure enough, the item was as described and I proceeded to clean the lens.

And so I opened up the rear assembly - no problems. Removed the offending element and cleaned it. Crystal clear now, so I re-installed. Time to shoot.

Oh dear - problems. The lens was 'focusing' but two issues emerged.

1) the images were terrible. Out of focus badly.
2) Won't go below f4.5 as displayed on the camera body ('A' lens)

So, I opened it up again, checked everything. Everything was tight and as should be. Cemented elements look good.

I tried phase and contrast detect - both yielded the same fuzzy image. Back/front focus is not evident.

Hmmm I can 'live' with (2) but (1) is a serious problem.

I should mention that IQ improves (and becomes acceptable but below bokina standards) with stopping down. This tells me that improvement with increased DoF may indicate registration distance issue????

Can anyone share some insight here to help me out.

Thanks in advance.


Last edited by Wild Mark; 02-20-2015 at 10:00 PM.
02-20-2015, 11:10 PM   #2
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It sounds like you have flipped an element when putting them back. try to find the optical design of the lens and check if any of them misassembled.
Good luck!
02-21-2015, 12:36 AM   #3
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Thanks for the tip. If I was so lucky! The element removal is so simple it is not funny.

There are three groups of elements, the front, middle and rear. They are removed complete in metal housing - I did not atomise any further (no need). So, unfortunately this is not a solution.

But, in saying this, I will check to see if someone else has made that mistake in the past.
02-21-2015, 05:55 AM   #4
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QuoteOriginally posted by Wild Mark Quote
1) the images were terrible. Out of focus badly.
2) Won't go below f4.5 as displayed on the camera body ('A' lens).
BUT, just how was the ~bokeh~ ???

;-)

02-21-2015, 05:59 AM   #5
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haha - it was fine, but lonely. I am now missing the sharpness. Doh
02-21-2015, 06:43 AM   #6
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QuoteOriginally posted by Wild Mark Quote
haha - it was fine, but lonely. I am now missing the sharpness. Doh
I asked technician to clean my Tokina 90 2.5. So I cannot give suggestion. But it sounds there is a decentering issue due to alignment.
02-22-2015, 01:38 AM   #7
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QuoteOriginally posted by starjedi Quote
I asked technician to clean my Tokina 90 2.5. So I cannot give suggestion. But it sounds there is a decentering issue due to alignment.
In short, maybe.

The front, middle and rear element assembly's independently function as expected. I can get crisp hand held focus with macro detail. I have compared performance with my Viv 90/2.5 to verify.

It is together that the elements don't perform properly. I suspect the inter assembly spacing is wrong, specifically the spacing between the front and middle.

Time to send to a technician.
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